Optical absorption spectroscopy is fundamentally important in chemical analysis, providing decisive quantitative and qualitative information. Such diagnostic capabilities find substantial utility in both research and industrial process environments. Therefore, an advancement in sensitivity, accuracy, or adaptability of the technique will have a significant impact.
Absorption is usually determined from measurement of a ratio of optical powers at a certain wavelength. Recently, a new technique, termed cavity ring-down spectroscopy (E. K Wilson, C&E News, Feb. 19, 1996, p. 34, incorporated herein by reference), has been developed to determine absorption by gases, which utilizes a pulsed light source and an optical cavity. Typically, a light pulse from a laser source is injected into a cavity which is formed by two high-reflectivity mirrors. The lifetime of the pulse in the cavity is highly sensitive to cavity losses, including absorption by gases. Measurement of the pulse decay time or "ring-down" time in the cavity can thereby provide a direct measure of absorption. Cavity ring-down eliminates the adverse effects of light source fluctuation, since the measurement is acquired with a single pulse of light. The feasibility of this technique arises from recent technological advances in optical polishing, which permit the fabrication of extremely low-scatter-loss optics. If ordinary optics such as high-reflectivity mirrors (R.about.99%)are used, the pulse lifetime in the cavity is too short for the cavity ring-down strategy to provide a significant improvement in sensitivity, as compared to conventional absorption methods. However, with the advent of superpolishing, such as that described in N. J. Brown, Ann. Rev. Mater. Sci. 16, p. 371 (1986), incorporated herein by reference, mirrors with 99.99% reflectivity or better can be fabricated to construct low-loss optical cavities, thereby permitting ultra-high sensitivity to be routinely realized. The cavity ring-down technique has thereby become a viable form of optical absorption metrology, with trace analysis capabilities that greatly exceed conventional absorption methods.